254 BIRKELAND. THE NORWEGIAN AURORA POLARIS EXPEDITION, 1902 1903. 



all powerful during this period. It is evident, however, from the conditions at the arctic stations, 

 especially Kaafjord and Matotchkin Schar, that it cannot be regarded only as an increase of the principal 

 storm, for the horizontal component of the perturbing force during this period turns round in the opposite 

 direction. 



Chart IV shows the current-arrows answering to the total perturbing force at i8 h 34 m . 



The current-system on the whole bears a fairly close resemblance to that of the principal storm, 

 which has already been described. 



The chief difference between them is that at Kaafjord and Matotchkin Schar the direction of the 

 force is the reverse of that which we find during the succeeding part of this section, as the current-arrow 

 is now directed along the auroral zone from west to east. The magnitude of the total perturbing force at 

 Matotchkin Schar now gives a false impression of the forces that are in operation, as the total force 

 there seems to be about equal to the difference of the forces actually present. At Kaafjord, however, 

 the long principal storm, with current-arrows directed westwards, does not seem to have any noticeable 

 influence until about ig h 3O m . 



As regards Matotchkin Schar, we find that the current-arrows again point in the direction they had 

 in the first section. 



Unlike the distribution of force during the principal perturbation, the current-arrows in Europe are 

 now directed westwards, and at the most northerly stations even a little north. These last, during the 

 principal perturbation of the third section, had a more southerly direction. 



In America the conditions are essentially the same as those during the long perturbation, the only 

 exception being that the arrow at Sitka is comparatively longer and more eastward in direction: 



We thus see that this time also, the perturbing forces approximately diverge from a point in the North 

 Atlantic. The strength with which the perturbation appears in the regions round Batavia, Dehra Dun 

 and Zi-ka-wei is especially worthy of notice. 



The arrows at Irkutsk, Honolulu and Sitka indicate the formation of negative vortices corresponding 

 to a convergence of the perturbing forces. In this case, the area of convergence would be situated in 

 the regions surrounding the Behring Sea. 



The second intermediate storm, from ao h 45 to 2i h 2o m . 



In the decomposition of the total perturbing force in this storm, we have attempted to distinguish 

 between its effect and that of the principal storm, "at all the southern stations where the conditions before 

 and after are constant. 



At the arctic stations the curve shows distinctly that a particularly strong impulse occurs during 

 this period, especially noticeable at Axeleen, where the surrounding conditions are fairly normal. 



We have therefore not thought it advisable to undertake any decomposition there. The normal 

 line for the taking out of the partial part, should be the curve as it would be drawn on paper if the 

 principal storm only had been acting; but owing to the rapid change in the principal perturbation, 

 this line cannot be determined with sufficient certainty. 



The result of the decompositions is shown on Charts VII X. The resulting arrows are here 

 drawn entire. The arrows representing the principal storm are drawn with a dotted line, those repre- 

 senting the intermediate storm with a broken line. 



The field in the principal storm is of course the same as that previously described. 



In the field of force and its variations, this intermediate storm shows a great resemblance to the 

 ordinary polar elementary storms, such as those of the I5th December, 1902, the ioth February, 1903, etc. 



On Chart VIII - - for 2o h 52.5 m - - the partial current-arrows in the district Pawlowsk to San 

 Fernando are directed south-east, and at Tiflis east, while farther east they turn more north. This 

 indicates a convergence of the perturbing force in the north-west of Asia or the north-east of Europe. 



